One particularly simple embodiment of connections for electrical appliances is plug contacts, in which there is only a force fit between a connecting contact on the appliance side, and a plug to be connected thereto. One advantage of plug contacts is that the electrical connection can be made simply by plugging the plug into the connecting contact, and can be disconnected again by pulling the plug out of the connecting contact. So-called fork or lyre contacts represent one form of plug contacts.
One example of a fork contact such as this according to the prior art is illustrated in the form of a schematic perspective view in FIG. 1. In the case of fork contacts, one side of the connection, preferably the connection on the appliance side, is formed by two contact clips 10, 12, which are arranged at a distance from one another. A so-called blade contact 14 of the further appliance can be inserted between the two connecting clips 10, 12, in order to make an electrical contact with a further appliance.
In order to make it easy to insert the blade contact 14 between the two contact clips 10, 12, particular shapes may be provided on contact areas 16 of the two contact clips 10, 12. In the example shown in FIG. 1, the contact clips 10, 12 are attached to a contact arm 18 of the electrical appliance, which is not illustrated. A total current G emitted from the electrical appliance flows via the contact arm 18 to the fork contact, where is it is split between the two contact clips 10, 12 and thus passed to the contact areas 16, forming two current paths which run in parallel. The two current elements merge at the contact areas 16 into the blade contact 14, thus once again resulting in the total current G in the blade contact 14, which is then carried away in the blade contact 14.
FIG. 1 shows only parts of the contact arm 18 and of the blade contact 14, as a result of which neither the source of the total current G nor its sink can be seen.
The total current G should be transferred from the contact arm 18 to the blade contact 14 with losses that are as low as possible. Particularly in the event of a short circuit, when the total current G transmitted via the fork contact and therefore also the transmitted electrical power are above the maximum permissible level, it is of major importance for the proportion of this transmitted power which was produced as lost power in the fork contact to be sufficiently low but the fork contact is not damaged by heating in the event of a short circuit. In this context, particularly the contact surfaces of the contact areas 16 of the two contact clips 10, 12 with the blade contact 14 have a tendency not to rest completely on the blade contact because of uneven areas on the surfaces of the contact areas 16, with only subregions allowing the total current G to be transmitted from the contact clips 10, 12 to the blade contact 14. Electrical losses lead to heating at such current constrictions, where the total current passes through a relatively small cross section.
In addition, forces F, F′ occur at contact constrictions and force the contact areas 16 of the contact clips 10, 12 away from the blade contact 14. Forces such as these are referred to as current constriction forces or Holm forces, and result in constrictions of current paths. Current constriction forces are Lorentz forces which are formed on both sides of a constriction of a current path, because of currents running in opposite directions. If current constriction forces F, F′ lead to the contact areas 16 being moved away from the blade contact 14, the current transmission areas are constricted further. In consequence, the current constriction forces F, F′ increase further resulting first of all in one of the two contact clips 10, 12 lifting off. Since the total current G is now carried completely by the contact clip which is still in contact, the current constriction forces F, F′ on its contact area 16 increase once again, and the remaining contact clip 10, 12 is also disconnected from the blade contact 14. This leads to arc formation, heating of the fork contact and (in the worst case) to fusing of the contact areas 16 to the blade contact 14, that is to say to destruction of the fork contact.
Since the current constriction areas on the contact areas 16 of each contact clip 10, 12 are different, different contact resistances are formed between the blade contact 14 and a respective contact clip 10, 12, as a result of which the total current G is not distributed uniformly between the two contact clips 10, 12 but, in some circumstances, one of the two contact clips 10, 12 carries a greater current. It has been found from investigations that, in extreme cases, it is possible for one of the contact clips 10, 12 to carry up to 80% of the total current G, while only the remaining 20% of the total current G flows in the second contact clip 10, 12. In a corresponding manner, greater current constriction forces F, F′ act from the start on the contact clip 10, 12 carrying the greater proportion of the current. In consequence, the process as just described of disconnecting one contact clip first of all followed by subsequent disconnect of the remaining contact clip, with the described destructive consequences, is assisted by the asymmetric current distribution.
The occurrence of current constrictions in the contact areas 16 can be reduced by forcing the two contact clips 10, 12 toward one another, whilst resulting in a pressure force of the contact areas 16 on the blade contact 14, and thus improving the electrical contact between the contact areas 16 and the blade contact. However, the two contact clips 10, 12 cannot be forced toward one another until the blade contact 14 has been inserted since, otherwise, the insertion process would itself be made more difficult. In this context, fork contacts have the advantage that the required pressure force is produced by the total current G itself: since the total current G is carried through the two contact clips 10, 12 on two current paths which run in parallel, this results in a magnetic field H which surrounds the two contact clips 10, 12 and which in turn results in a force which forces the two contact clips 10, 12 toward one another in the desired manner.
The force produced by the field H which surrounds the two contact clips 10, 12 is increased, according to the prior art, by placing two magnetically permeable brackets 20, 22 around the two contact clips 10, 12. The two brackets 20, 22 are separated from one another by two air gaps 24, 26. Magnetically permeable brackets should in the present case be understood as meaning that these brackets are manufactured at least partially from a material having high magnetic permeability (preferably with a relative permeability of more than two). Magnetically permeable elements are preferably manufactured from ferromagnetic material, in particular so-called construction steel (steel 1010).
In order to explain the effect of the two brackets 20, 22, FIG. 2 shows a section through the arrangement illustrated in FIG. 1. The section in this case runs along the line II-II shown in FIG. 1. FIG. 2 therefore shows cross sections through the two contact clips 10, 12 and cross sections through the two magnetically permeable brackets 20, 22. The total current G in the contact clips 10, 12 is split into two current elements I, I′ flowing in a direction at right angles to the plane of the drawing in FIG. 2, as indicated by vertical direction arrows (circles with dots in them). The magnetic field H formed by the two current elements I, I′ causes a magnetic flux B in the interior of the brackets 20, 22, which magnetic flux B appears as a magnetic flux density field B′ in the gaps 24, 26 as it passes between the brackets 20, 22. The flux density field B′ which is formed between the two boundary surfaces of a gap 24 and 26 results in a force on these surfaces, that is to say the two brackets 20, 22 attract one another, attempting to close the gaps 24, 26. Since the two contact clips 10, 12 are resting on the brackets 20, 22, the magnetically permeable brackets 20, 22 will result in an additional force, which forces the two contact clips 10, 12 together, being present when a current flows.
Since the flux density field B′ running in the gaps 24, 26 is formed only when the fork connection is carrying current, the two brackets 20, 22 are attracted to one another only when current is actually flowing via the fork connection. It is therefore possible to easily insert a blade contact into the fork connection, and to detach it therefrom again, when the appliances are switched off. Since the two brackets 20, 22 would have fallen away from the contact clips 10, 12 when no field B′ is present, they are mechanically secured by a holding clip 28.
Inter alia, the force acting on the contact clips 10, 12 as a result of the apparatus comprising the two magnetically permeable brackets 20, 22 is highly dependent on the width of the gaps 24, 26, that is to say on the distance between the two brackets 20, 22. This has the disadvantageous consequence that the gaps 24, 26 are also at the same time enlarged in the situation when one of the contact clips 10, 12 has been raised slightly off the blade contact 14 as a result of a current constriction force F, F′. Since this results in a reduction in the forces between the brackets 20, 22, that is to say the force counteracting the current constriction force F, F′ is decreased, the contact clip which is initially only slightly raised is forced further away from the blade contact 14, and the destructive consequences that have already been described can occur. In this case as well, the total current G is once again split asymmetrically into the two current elements I, I′ in the contact clips 10, 12, in a particularly disadvantageous manner.
It is known from simulations that, with the described fork contact from the prior art, and when the total current G is split asymmetrically, a total current level of more than 30 kA can lead to the described destructive effects of the current constriction forces F, F′. In the case of relatively large electrical appliance, particularly in the field of appliances with a low-voltage supply and correspondingly high operating currents, a total current of more than 30 kA can occur, however, in the event of a short circuit, before circuit breakers interrupt the short-circuit current. It is therefore possible in a situation such as this for a fork contact according to the prior art to be destroyed.